This invention relates to temperature actuated variable neutral density filters and more particularly to liquid crystal dispersions which are formed into a film and used on a variety of surfaces and in combination with polarizers and/or half wave and/or quarter wave length plates for reflecting or transmitting the visible and infrared spectrum in accordance with the temperature of the film in which the cholesteric liquid crystal micron size dispersion called micelles are embedded.
Liquid crystals are materials whose molecules arrange themselves in ordered groupings having many characteristics of crystals. As an example, a crystalline solid such as diamond or table salt is characterized by molecules having both positional and orientational order. A simple crystalline solid has a single repeating molecule or unit which is placed or oriented with respect to a neighboring unit with the sequence repeated over and over which repetition and orientation is responsible for the crystalline state. If the crystalline solid loses both its positional and orientational order, the solid converts into an amporphous order which is easily brought about by heat. The degradation of the crystal into an amorphous mass proceeds in stages. The first stage is loss of either positional or orientational order with the next stage being the loss of both. Materials which retain the orientational but lack the positional order are liquid crystal materials. An essential characteristic of the liquid crystal molecules are that they all point in the same direction and are characterized by being long and narow. Liquid crystals fall into three classes, nematic, smectic, and cholesteric which all share a common characteristic of orientational molecular order. Nematic liquid crystals have molecules all pointing more or less in the same direction and all having their long axis parallel to each other but free to move in any direction. In the smectic liquid crystalline phase the molecules are aligned in parallel layers with any two layers free to slide over each other as long as the individual molecules do not move out of their layers. In the cholesteric phase molecules slide parallel in layers with the alignment axis shifting in each successive layer so that a helix is traced out through the various layers. The aforesaid liquid crystal phases exist only over a limited range of temperatures and below such temperatures the liquid crystal may become a solid with crystalline properties and above it the material may cease to be a crystalline liquid and may become an ordinary liquid with isotropic properties.
The present invention is interested in the characteristics of thin films of cholesteric liquid crystals which exhibit a property upon the interaction with light which may be termed selective scattering. Scattering is used rather than reflection in order to distinguish from the effect occurring on a mirrored surface wherein light is reflected at an angle equal to the angle of incidence. In viewing the light impinging on a film on the same side at which it is viewed, a selectively scattering film has an apparent color which is the complement of the color of the light transmitted by the film. In other words, the light scattered by the film is within a wave length band which if added to the transmitted light through the material would form white light if white light is what was incident on the film. Most materials do not selectively scatter light in that the light transmitted through them is not complementary to the light scattered by them but rather is within the same wave length band. The phenomenom of selective scattering exhibited by cholesteric liquid crystal film is independent of whether the light illuminating the film is polarized or not and the color and intensity of the scattered light depends upon the temperature of the scattering material and upon the angle of incidence of the illumination.
If a cholesteric liquid crystalline film is illuminated with circularly polarized light in addition to selective scattering, it also exhibits the property of circular dichroism which may be defined as differential interaction of a material upon circularly polarized light of different senses. Unpolarized light comprises right and left handed circularly polarized components of equal intensity. Upon interaction with a film of circularly dichroic material the right handed component is transmitted with a different intensity than the left handed component. The extent of the difference is a function of the temperature in the cholesteric liquid crystalline material. The cholesteric liquid crystal material also exhibits optical activity which may be observed by illuminating the material with linearly polarized light. Due to the optical activity of the material the polarization vector of the light is caused to rotate and the amount of this rotation is dependent upon the temperature of the material.
These particular characteristics of cholesteric liquid crystalline material have been used, for example, for converting thermal patterns to visible patterns, for indicating temperatures and in many forms of display as well as a variety of decorative articles. As such these uses are limited and specific.
Coated glass and plastic products selectively filter light and as such do not really control the brightness of the complete light spectrum, but give off a characteristic color corresponding to that color which is reflected or otherwise filtered out by the coating which has been applied to the transparent surface. Accordingly, it would be useful to be able to control the entire spectrum of the light such that it appears monochromatic and not colored or metallic, and further so that it can be used selectively to reflect or scatter infrared for passing visible light independently.